1. Field
The present embodiments generally relate to the gasification of hydrocarbons. More particularly, embodiments of the present invention relate to recovering heat from syngas and power produced therefrom.
2. Description of the Related Art
Gasification is a high-temperature process usually conducted at elevated pressure to convert carbon-containing materials into carbon monoxide and hydrogen gas. Since this gas is often used for the synthesis of chemicals or synthetic hydrocarbon fuels, the gas is often referred to as “synthesis gas” or “syngas.” Syngas can be used as a fuel to generate electricity or steam, as a source of hydrogen, and as a raw material in the production of fuels, chemicals, fertilizers, and industrial gases.
There are three basic gasifier types: fixed bed, fluidized bed and entrained flow. The fixed bed gasifier is operated at relatively low outlet temperature of about 425° C. (797° F.) to about 600° C. (1,112° F.) and requires a lesser amount of oxygen compared to the other two types of gasifiers; however, the product syngas contains substantial unconverted methane, and by-product tars and oils. The fluidized bed gasifier operates at more moderate outlet temperatures of about 900° C. (1,652° F.) to about 1,050° C. (1,922° F.) and requires a greater amount of oxygen than a comparable fixed bed gasifier. While the syngas from a fluidized bed gasifier is of higher purity, the carbon conversion is lower than a comparable entrained flow gasifier which operates at much higher temperatures of about 1,250° C. (2,282° F.) to about 1,600° C. (2,912° F.) and requires significantly higher energy input, but from which syngas of the highest purity can be obtained.
The high temperature in the entrained gasifiers and in the lower zones of certain fixed bed gasifiers converts the inorganic materials in the feed into a molten vitrified material which solidifies when removed from the gasifier, producing a material resembling coarse sand and generally referred to as slag. Fluid bed gasifiers produce dry ash which is not vitrified but only consolidated or agglomerated. Depending on the gasifier, it is desirable either to remove ash at lower temperatures (non-slagging gasifiers) or as a low viscosity liquid at high temperatures (slagging gasifiers). This inert slag or ash has a variety of uses in the construction and building industries.
The raw syngas can be treated using proven commercial technologies to remove trace elements and other impurities for recovery or recycle to the gasifier. Sulfur can be recovered as marketable elemental sulfur or sulfuric acid. In order to remove the contaminants from the syngas, the most economically feasible means is to cool the syngas first. One conventional technique used to cool the syngas is direct cooling, in which a quench fluid is introduced to the syngas. Another technique is indirect cooling where saturated steam is generated. This saturated steam is generally further heated, requiring additional fuel or other source of heat, in a heat recovery system to generate superheated steam for use in generating power in a steam turbine.
There is a need, therefore, for more efficient systems and methods for utilizing heat generated in producing syngas.